Temperature stabilized resonator

ABSTRACT

A hollow compensation member is secured in a wall of the cavity resonator in such a manner as to project inwardly, said compensation member having a wall thickness in the cavity which is less than that of the cavity resonator structure and being made of a material having a lower coefficient of linear thermal expansion than the material forming the structure of the cavity resonator.

United States Patent [191 Dorsi et al.

1 Mar. 25, 1975 I TEMPERATURE STABILIZED RESONATOR [75] Inventors: Domenico Dorsi; Marco Panigata,

both of Milan, Italy [73] Assignee: GTE International Incorporated,

Stamford, Conn.

[22] Filed: July 30, 1973 [21] Appl. No.: 383,940

[30] Foreign Application Priority Data OTHER PUBLICATIONS Brady; M. M., Simple Temperature Compensation of Large Rectangular Wave Guide Resonant Cavities," Electronic Engineering, 2-1969, pp. 198-199.

' Primary ExaminerJames W. Lawrence Assistant Examiner-Wm. H. Punter Attorney, Agent, or FirmGTE International Incorporated [57] ABSTRACT A hollow compensation member is secured in a wall of the cavity resonator in such a manner as to project inwardly, said compensation member having a wall thickness in the cavity which is less than that of the cavity resonator structure and being made of a material having a lower coefficient of linear thermal expansion than the material forming the structure of the cavity resonator.

12 Claims, 3 Drawing Figures TEMPERATURE STABILIZED RESONATOR BACKGROUND OF INVENTION This invention relates to waveguide microwave filters with cavity resonators and more particularly to an arrangement for the temperature stabilization of the resonant frequency of cavity resonators in such filters.

In radio links, a minimization of the frequenccy shifting, due to temperature changes, of the passband of the waveguide filters is required. This is done in order to limit the deterioration in the quality of the communications, which would otherwise result therefrom.

One method of reducing passband frequency drift caused by changes in ambient temperature consists of forming the filter resonators by means of materials having a low coefficient of linear thermal expansion. Waveguides made of materials having a low coefficient of linear thermal expansion are expensive, however, and their low expansion characteristics deteriorate when they are heated to high temperatures, such as for example the temperatures that are employed in hard soldering.

Other known methods, for example, are those described in the following publications: Stabilizzazione termica di alcuni tipi di filtri a microonde (Temperature stabilization of certain types of microwave filters), presented by D. Dorsi and M. Panigata at the LXXI Annual Meeting of AEI, (Association Elettrotecinica Italiana) and Technique of Microwave Measurements by C. G. Montgomery, MIT Rad. Lab Series, McGraw-l-Iill Book Co., 1947. These methods, however, lead to some mechanical complications.

An object of this invention is the provision of a temperature stabilized microwave cavity resonator that is economical and of a simple structure.

Another object of this invention is to provide a mechanical arrangement for the temperature stabilization of the resonant frequency of a microwave cavity resonator, wherein said arrangement, particularly as applied to waveguide microwave filters of the type haviing direct coupled cavity resonators, in addition to reducing the frequency drifts due to ambient temperature changes, provides the advantages of a lower cost with respect to the use of waveguides made of materials having a low coefficient of linear thermal expansion and a lower constructional complication with respect to other methods described in the above-mentioned literature.

SUMMARY OF INVENTION These objects are achieved in accordance with this invention by an arrangement characterized in that a hollow compensation member is secured in a wall of the cavity resonator in such a manner as to project inwardly, said compensation member having a wall thickness in the cavity which is less'than that of the cavity resonator structure and being made of a material having a low coefficient of linear thermal expanison than the material forming the structure of the cavity resonator.

BRIEF DESCRIPTION OF DRAWING This invention will be better understood from the following detailed description of preferred embodiments thereof, given merely by way of example and therefore in no limiting sense, referring to the accompanying drawing, wherein:

FIG. 1 shows a sectional view of a cavity resonator with the arrangement according to the invention;

FIG. 2 is a plot showing the change of the cavity resonant frequency as a function of a dimensional magnitude of the arrangement in FIG. 1 according to the invention; and

FIG. 3 is-a longitudinal section view of an alternate embodiment of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the preferred embodiment of this invention in FIG. 1, there is shown a sectional view of a cavity resonator 1 of a cavity resonator microwave filter, the latter not further illustrated. The ends of the rectangular cavity 1 may be closed by structure such as a solid wall, posts, windows, etc. The cavity 1 supports propagating electromagnetic waves. Apparatus for conducting electromagnetic signals to and from the cavity are well known and are therefore not shown in the drawing. The walls surrounding the cavity are a ground plane. A circular aperture 2 is provided at the middle of one of the broad walls of the cavity 1 (that can be made of brass or copper) and a compensation member in the form of a cup 3 of invar (eutectic comprising 64% iron and 36% nickel) is inserted in a sealing relationship into the aperture 2 with its bottom projecting inwardly into the cavity. This cup bottom is shaped as a cone 4 which has its base substantially in the plane of the internal surface of the cavity wall and has a height h and thin wall of thickness s. The cup, with an internal diameter d), is closed by a plug 5 of the same material as the walls of cavity resonator 1 and shaped as a right circular cylinder. The assembly thus obtained is forced into the apparatus 2. The mechanical couplings between parts 1, 3, and 5 are then reinforced by soldering along all of the coupling surfaces. The walls of cavity 1 may be made of brass or copper. The cup 4 is preferably made of a material such as invar (eutetic comprising 64% iron and 36% nickel) which has a lower coefficient of linear thermal expansion than the walls of cavity 1.

In opration, it is well known that by introducing into a cavity resonator a member of a suitable material and shape, the electrical features and therefore the resonant frequency of the cavity can be changed. More particularly, the resonant frequency (f of the cavity resonator l at a given temperature is related to the size h of the compensation member 3, that is the height of the cone 4, according to a law which is qualitatively represented in FIG. 2. Assume that a temperature increase AT is applied to the cavity 1 shown in FIG. 1. In the absence of the compensation member 3 the resonant frequency would tend to change according to a relation of the following type:

of the material forming the cavity (brass or copper).

The coefficient of linear thermal expansion of invar is much smaller than the coefficient of linear thermal expansion of brass and copper. If the wall thickness s of the cone-shaped bottom 4 of the compensation member is sufficiently small, as the temperature increases the height h decreases. This effect, as seen in FIG. 2, tends to cause the resonant frequency of the cavity to increase in opposition to the natural resonant frequency (see equation (1)). A similar discussion may be made. in the case of a temperature decrease.

By suitably sizing the height h, the diameter and the thickness s of the invar cup bottom, the two changes described above can be caused to exactly compensate. The dimensions of a temperature compensated cavity resonator embodying this invention that was built and tested and which operated satisfactorily were:

Cavity l length: 48 mm (f -4250 MHz) Cavity l height: 29.083 mm (WR 229 waveguide) Cavity l width: 58.17 mm h 0.65 mm s 0.2 mm (b 28 mm A longitudinal section view of a stripline resonator embodying this invention is shown in FIG. 3. This resonator comprises dielectric slabs 7 and 8 sandwiched between associated parallel plates 9, l0, and 11. The dielectric is cut away adjacent the bottom 4' of the cup 3.

While but two preferred embodiments of the invention have been illustrated and described, it is obvious that many changes and modifications can be made without departing from the scope of the invention. By way of example, this invention may also be employed in slab line, microstrip, and coaxial resonators, to mention only a few alternate embodiments. Also, the bottom of the cup may have a hemispheric or polyhedron shape.

What is claimed is:

1. A temperature-compensated cavity resonator comprising a circuit for supporting electromagnetic fields, in-

cluding an electrically conductive ground plane having an aperture therethrough,

a hollow cup-shaped thermal compensation member sealing said aperture and having a bottom extending through said ground plane into the area of electromagnetic fields, the thickness of said bottom of said cup being less than that of said ground plane, said cup member being made of a material having a lower ccoefficient of linear thermal expansion than the material forming said ground plane, and

a plug of the same material as said ground plane closing said cup and being substantially aligned in said cup with said ground plane.

2. The resonator according to claim 1 wherein said bottom of said cup has a base substantially aligned with the surface of said ground plane proximate to, and has a surface area projecting beyond said base into, the area supporting electromagnetic fields.

3. The resonator according to claim 2 wherein said ground plane is flat.

4. The resonator according to claim 2 wherein said ground plane is a rectangularly shaped conductive member.

5. The resonator according to claim 1 wherein said aperture is circular and said bottom is cone shaped.

6. The resonator according to claim 1 wherein said aperture is circular and said bottom is hemispherically shaped.

7. The resonator according to claim 1 wherein said base is polygon shaped and said projecting bottom is a polyhedron with a plurality of sides having a common point.

8. An arrangement for the temperature stabilization of the resonant frequency of a microwave cavity resonator, particularly for microwave filters with direct coupled cavity resonators, wherein the cavity resonator is enclosed into a structure having a given coefficient of linear thermal expansion, characterized in that a ho]- low compensation member is secured to a wall of the cavity resonator in such a manner as to project inwardly; said compensation member having a thickness which is smaller than the thickness of the cavity resonator structure and being made of a material having a lower coefficient of linear thermal expansion than the material forming the structure of the cavity resonator; and characterized in that a circular aperture is provided in a wall of the cavity resonator structure and that the hollow member is in the form of a cup sealing said aperture, said cup having its bottom projecting inwardly into the cavity, and being closed by a plug of the same material as the cavity resonator structure substantially aligned with the cavity resonator wall in which the aperture is provided.

9. An arrangement as set forth in claim 8, characterized in that the cup bottom projecting inwardly into the cavity resonator is cone-shaped.

10. An arrangement as set forth in claim 8, characterized in that the cup is forced into the aperture of the cavity resonator wall and the plug is in turn forced into the cup.

11. An arrangement as set forth in claim 10; characterized in that the cup is soldered both to the surface of the aperture in the cavity resonator and to the plug closing the cup. I

12. An arrangement for the temperature stabilization of the resonant frequency of a microwave cavity resonator structure having a given coefficient of thermal expansion comprising: a hollow cup-shaped compensation member sealing an aperture in a wall of the cavity resonator; said compensation member having a bottom projecting inwardly into the caivty, having a bottom wall thickness in the cavity which is less than the thickness of the wall of the cavity resonator structure, and being made of a material having a coefficient of thermal expansion that is different from that of the material forming the cavity resonator structure; and a plus that is made of the same material as the cavity resonator structure closing said cup and being substantially aligned with the apertured wall of the cavity resonator structure.

UNITED STATES PATENT OFFICE CERTIFICATE OF CGRECTIGN PATENT NO. 3,873,949

DATED March 25, 1975 INVENTOR( I Domenico Dorsi & Marco Panigata It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1 line 9, "frequenccy" should read frequency same column 1, line 32, "Elettrotecinica" should read Elettrotecnica same column 1, line 65, "low" should read lower Column 2, line 49, "opration" should read operation Column 3 (claim 1), line 52, "ccoefficient" should read coefficient Column 4 (claim 11) line 42, after "10'', the semicolon should be changed to a comma; same column 4 (claim 12) line 52, "caivty" should read cavity same column 4 (same claim 12) line 57, "plus" should read plug Signed and Sealed this second Day Of September 1975 [SEAL] Arrest."

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner ()[PaMnts and Trademarks 

1. A temperature-compensated cavity resonator comprising a circuit for supporting electromagnetic fields, including an electrically conductive ground plane having an aperture therethrough, a hollow cup-shaped thermal compensation member sealing said aperture and having a bottom extending through said ground plane into the area of electromagnetic fields, the thickness of said bottom of said cup being less than that of said ground plane, said cup member being made of a material having a lower ccoefficient of linear thermal expansion than the material forming said ground plane, and a plug of the same material as said ground plane closing said cup and being substantially aligned in said cup with said ground plane.
 2. The resonator according to claim 1 wherein said bottom of said cup has a base substantially aligned with the surface of said ground plane proximate to, and has a surface area projecting beyond said base into, the area supporting electromagnetic fields.
 3. The resonator according to claim 2 wherein said ground plane is flat.
 4. The resonator according to claim 2 wherein said ground plane is a rectangularly shaped conductive member.
 5. The resonator according to claim 1 wherein said aperture is circular and said bottom is cone shaped.
 6. The resonator according to claim 1 wherein said aperture is circular and said bottom is hemispherically shaped.
 7. The resonator according to claim 1 wherein said base is polygon shaped and said projecting bottom is a polyhedron with a plurality of sides having a common point.
 8. An arrangement for the temperature stabilization of the resonant frequency of a microwave cavity resonator, particularly for microwave filters with direct coupled cavity resonators, wherein the cavity resonator is enclosed into a structure having a given coefficient of linear thermal expansion, characterized in that a hollow compensation member is secured to a wall of the cavity resonator in such a manner as to project inwardly; said compensation member having a thickness which is smaller than the thickness of the cavity resonator structure and being made of a material having a lower coefficient of linear thermal expansion than the material forming the structure of the cavity resonator; and characterized in that a circular aperture is provided in a wall of the cavity resonator structure and that the hollow member is in the form of a cup sealing said aperture, said cup having its bottom projecting inwardly into the cavity, and being closed by a plug of the same material as the cavity resonator structure substantially aligned with the cavity resonator wall in which the aperture is provided.
 9. An arrangement as set forth in claim 8, characterized in that the cup Bottom projecting inwardly into the cavity resonator is cone-shaped.
 10. An arrangement as set forth in claim 8, characterized in that the cup is forced into the aperture of the cavity resonator wall and the plug is in turn forced into the cup.
 11. An arrangement as set forth in claim 10; characterized in that the cup is soldered both to the surface of the aperture in the cavity resonator and to the plug closing the cup.
 12. An arrangement for the temperature stabilization of the resonant frequency of a microwave cavity resonator structure having a given coefficient of thermal expansion comprising: a hollow cup-shaped compensation member sealing an aperture in a wall of the cavity resonator; said compensation member having a bottom projecting inwardly into the caivty, having a bottom wall thickness in the cavity which is less than the thickness of the wall of the cavity resonator structure, and being made of a material having a coefficient of thermal expansion that is different from that of the material forming the cavity resonator structure; and a plus that is made of the same material as the cavity resonator structure closing said cup and being substantially aligned with the apertured wall of the cavity resonator structure. 